Bat detector

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Bat detector (signal: 31.8 kHz )
Audio sample (68 kB): Pipistrelle calls in the heterodyne detector, immediately before catching prey.

The bat detector is an electronic device that converts the ultrasonic sounds of bats into sounds that can be heard by humans. To locate prey, for orientation or to communicate with conspecifics, bats emit sounds that are predominantly in the ultrasonic range. The bat detector is used to track down bats and to better identify their species. The field of application of the bat detector is not limited to research on bats. Any other natural or artificial ultrasound source in the relevant frequency range can be detected and, if necessary, analyzed. The frequency range covered by a bat detector varies. The lower limit goes down into the audible frequencies - depending on the individual hearing ability. The lower limit of a bat detector is around 10 to 20 kHz. The upper limit of a bat detector is 100 kHz up to about 250 kHz.

The versions of the bat detectors range from self-made, simple kits and simple ready-made devices to professional high-tech devices that cost several thousand euros.

Functional units of the bat detector

microphone

Commercially available microphones or special ultrasonic microphones are used . Ultrasonic receivers, such as those used in ultrasonic distance measuring devices, are also suitable to a limited extent.

Preamplifier

The preamplifier amplifies the weak electrical signal from the microphone. Ideally, the preamplifier is dimensioned so that it compensates for the frequency response of the microphone used.

Signal conversion into audible tones

This is where the central functional unit of the bat detector is located. Various methods are used to convert the frequencies that are inaudible to humans into lower frequencies.

Basic procedures for converting the signals

Frequency divider

The signal is divided by an integer factor with a digital frequency divider ( synchronous counter , asynchronous counter ). A frequently used division factor is 1:10. A bat call with 40 kHz is reproduced after division by 10 as a sound with 4 kHz. A disadvantage of the simple frequency division is that the information about the amplitude (volume) of the tone is lost. The amplitude of the digital output signal is independent of the incoming amplitude. The information can be recovered by amplitude modulating the digital signal with the envelope of the input signal. One advantage of the frequency divider is that a large bandwidth can be monitored simultaneously.

Other common division factors are 1: 8, 1:16 and 1:32. These are common powers of two in digital technology . The division factors 1:16 and 1:32 seem more sensible here, as they divide the entire frequency range of interest into the audible range better than 1:10 or 1: 8.

Frequency mixer

In the frequency mixer, the incoming signal is mixed with a fixed frequency. The resulting signal is a multitude of sum and difference frequencies of which only the audible ones are reproduced. The structure of frequency mixers is relatively easy to implement. Here, too, a large bandwidth is monitored simultaneously. However, the rendering is patchy. If the resulting output signal is at very low frequencies, nothing can be heard. Example: A bat call of 41 kHz is heard as 1 kHz after mixing with 40 kHz. A bat call of 40.001 kHz would be played back as a one Hz signal after mixing, so it would not be audible.

Heterodyne receiver

In the heterodyne receiver (heterodyne receiver or superhet), the incoming signal is mixed with an adjustable frequency and then filtered. In the "double superhet" there are two mixing stages. The first with an adjustable frequency and the second with a fixed frequency (intermediate frequency).
The heterodyne bat detector works in principle like a radio receiver. You tune the detector to a certain frequency and only hear calls that are received in a narrow band around this tuned frequency. Calls that differ in frequency are suppressed by filtering. Only calls that are close together in frequency can be heard simultaneously. This is comparable to two overlapping radio stations that can be heard in a radio receiver at the same time. The heterodyne bat detector is suitable for determining the exact frequency of calls and thus making a species determination more precise. This is less suitable for general bat location because most of the frequencies are masked out. This is also an advantage in the case of annoying noises (e.g. from barbecues).

The process can be greatly simplified if the mixing is carried out according to the IQ process and mixed products are processed further by the sound card of a PC. The display can also be shown immediately as a spectrum . It is advantageous that the signals can be stored permanently in digital form.

Time stretchers

Comparison of the heterodyne method and time expansion. During the first 9.6 seconds of the recording, bat calls were made audible by mixing in a 45 kHz sine wave, then the last recorded 2.5 seconds are played back 20 times slower. The structure and frequencies of the respective calls can only be recognized in the time expansion.

The methods described so far convert the ultrasonic sounds without delay. This is different with the time stretching method. The sounds are recorded permanently or in an endless loop of a few seconds and only played back slowly for listening. The procedure is technically complex, but enables the most precise evaluation of the recorded calls. If the ultrasound sounds are slowed down by a factor of ten, then of course only one tenth of the duration of the calls can be reproduced with continuous monitoring. This method is therefore not suitable for continuous, uninterrupted observation in "real time". This can be remedied by a combination with a frequency divider or mixer, so that an interesting sequence can be saved at the push of a button if necessary. This can later be evaluated as required. Since suitable storage media are becoming increasingly cheaper, it is foreseeable that longer recordings will also be made possible with compact, mobile time-dilation bat detectors.

DSP detector

DSP (Digital Signal Processing, German: digital signal processing ) is a special case. With the DSP, the analog call signal is digitized and processed using mathematical algorithms so that the result signal is in the audible range. In theory, all of the conversion methods described so far can be implemented using digital signal processing.

Falsification of the sounds by the procedures

A falsification of the sounds in order to convert them into the audible range is imperative, because only the original signal, which is not audible to humans, is unadulterated. Each implementation procedure distorts the calls of the bats differently. With the heterodyne detector, the pitch of the playback also depends on the frequency set. A species determination based on the reproduced sounds requires experience. The delay-free processes (frequency divider, mixer and heterodyne) reproduce at least the rhythm of the sounds without distortion. The output signal of a frequency divider without amplitude recovery is sometimes perceived as unpleasant.

Recording and analysis of the sounds

Most detectors have an output for recording the signals. This is particularly useful with the Zeitstrengthener, since the sound signals can be analyzed using suitable software. This gives you additional options for addressing the species and information about behavior.

A recording in the field is usually made on memory cards . A compression-free recording is important. The sound signals are then later transferred to a computer with a sound card, with wav files being generated. Recently, real-time recording systems have also been used to document bats, which record sounds at high sample rates directly on a memory card (CF, SDHC) or hard drive (connected to a computer via USB). These are preferably also used in the context of environmental reports in automatic operation.

The sounds are analyzed by means of suitable programs, BatSound Pro being the most important. BatScan and the programs from the AviSoft group are also used. Some universities are also developing systems for the automatic determination of bat calls. Dr. Parsons (New Zealand; previously Bristol, UK) and Martin Obrist (Switzerland). The first such software available - bcDiscriminator - was available in 2007 for Central European species.

In the meantime, the researchers from Switzerland have developed a new bat detector (BATLOGGER). This makes it much easier to observe and identify the species of bats. In addition to the recording and storage of calls, information such as location or temperature is stored for each call. A supplementary compendium (BatEcho) makes it possible to listen to reference recordings and to access further information. Software (BatScope) was specially developed for the consistent storage of the recorded data.

Use in wind turbines

In the so-called nacelle monitoring, special bat detectors are used, which record the bat activity at the hub height of wind turbines over a longer period of time (several months). The recorded bat activity can then be used to calculate the probability of bats staying in the rotor area.

Individual evidence

  1. ↑ Bat research at the WSL . Website of the Federal Research Institute WSL. Retrieved October 5, 2011.
  2. ↑ Robert Brinkmann, Oliver Behr, Ivo Niermann, Michael Reich (eds.): Development of methods for investigating and reducing the collision risk of bats on onshore wind turbines. Cuvillier 2011, ISBN 978-3-86955-753-3 . Table of contents and excerpt

literature

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Web links

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